A flyback switching power converter is commonly used to charge mobile devices as the converter's transformer provides safe isolation from AC household current. To provide a power pulse to the load, a primary-side controller cycles a power switch coupled to a primary winding in a transformer. A magnetizing current through the primary winding ramps up to a peak value whereupon the primary-side controller opens the power switch. While the primary-side switch was closed, a secondary-side controller opens a synchronous rectifier (SR) switch coupled to a secondary winding in the transformer. The secondary-side controller then closes the synchronous rectifier switch after the primary-side controller opens the power switch. The magnetic energy stored in the transformer from the magnetizing current causes the secondary winding current to pulse high and then ramp down to zero as the magnetic energy is depleted.
The secondary-side controller requires a power supply voltage VCC for its operation. During normal operation, the output voltage delivered to the load provides a convenient power source from which the power supply voltage VCC may be derived. For example, the secondary-side controller may include a low-dropout (LDO) voltage regulator to derive the power supply voltage VCC from the output voltage. But at low values for the output voltage, the secondary-side controller drives the synchronous rectifier switch at low efficiency, which leads to thermal heating issues. To alleviate the thermal stresses of driving the synchronous rectifier switch during periods of low output voltage, the present assignee developed an alternative power source for the secondary-side controller that is active during low output voltage operation as disclosed in U.S. Pat. No. 8,964,421, the contents of which are incorporated by reference in their entirety. In particular, the drain voltage on the synchronous rectifier switch drives another voltage regulator (e.g., another LDO) to produce the power supply voltage VCC during low output voltage operation. Since the drain voltage on the SR switch transistor pulses in each power switching cycle, the additional LDO is configured to regulate the power supply voltage for the SR controller using this pulsing drain voltage. In contrast, the output voltage does not pulse in this fashion such that a more conventional LDO topology may be used for the LDO that receives the output to regulate the SR controller power supply voltage. To distinguish this additional regulator from the output voltage power supply voltage path, it may be denoted herein as the pulse linear regulator (PLR). Although the thermal issues are then mitigated, the PLR circuit can only charge the VCC capacitor while the primary switch is on. It is during this on-time that the drain-to-source voltage for the synchronous rectifier switch pulses above the output voltage. But during periods of low load, the power switch cycles on only briefly during relatively long switching periods (low duty cycle). The average current delivered to the VCC capacitor by the PLR circuit is proportional to the power switching duty cycle such that as the power switching duty cycle drops during periods of low load, the PLR circuit cannot maintain the power supply voltage VCC. The secondary-side controller then stops working such that the synchronous rectifier switch remains open regardless of the power switch cycling. The output current to the load is then delivered through the body diode of the synchronous rectifier transistor switch. This is problematic for primary-only feedback control of the output voltage by the primary controller. In particular, the primary controller samples the reflected voltage on the primary winding (or on the auxiliary winding) at the transformer reset time during primary-only feedback control of the output voltage. But the conduction through the body diode of the closed synchronous rectifier switch due to insufficient power supply voltage produces a diode voltage drop that prevents the primary-side controller from properly sampling the output voltage.
Accordingly, there is a need in the art for improved power supply voltage regulation for a secondary-side controller in flyback converters with synchronous rectification.